The present invention relates to scanning documents of large dimensions (or flat articles presenting little relief) at high resolution in order to enable them to be stored digitally in a computer system for subsequent archiving, consultation, reproduction, dissemination, or processing.
Until now, paper plans of large dimensions have been scanned using plan scanners, however they suffer from various limitations and drawbacks:
firstly, the maximum size of documents that can be scanned, which size is limited to standardized A0 format (840 millimeters (mm)xc3x971288 mm) for the largest devices available; and
scanning time which is of the order of 5 minutes for an A0 document, which time is prohibitive when a large corpus of documents are to be scanned, typically several thousands or several tens of thousands of paper plans.
One solution for scanning documents of size greater than A0 format consists in photographing the document by means of a large-format camera and in scanning the silver-based image obtained in this way using a photograph scanner.
That solution does indeed make it possible to mitigate the drawback of the limit on dimensions, however it is relatively impractical to implement and very expensive, and in addition it involves a large amount of handling time.
An object of the invention is to mitigate those various drawbacks by proposing document scanner apparatus which makes it possible simultaneously:
to accept documents of very large dimensions, significantly greater than A0 (it is shown below that it is easy to reach dimensions of 2000 mmxc3x973334 mm);
to produce very high resolution in the scanned image, typically of the order of 0.1 mm in the document to be reproduced;
to be very fast in use (typically requiring a few seconds or at most a few tens of seconds per document);
to deliver information that is directly in digital form and usable by a computer system without making use of a silver-based medium or some equivalent intermediary; and
to be capable of producing documents at will either with a gray scale, or as lines at higher resolution, or in color.
The invention is thus particularly suited to very high resolution scanning of a large corpus of paper documents where the documents are both large or very large in dimension and are present in very large numbers, such as collections of old maps which it is desired to have in digital form.
Furthermore, unlike plan scanners which are limited to reproducing documents that are made of paper and that are in good condition (so as to allow a document to be fed properly), the invention is applicable to reproducing paper documents that are in poor condition, have been retouched, corrected, repaired, or indeed to reproduce documents that are thick or articles that are flat but present little relief, or documents or articles that are fragile, insofar as reproduction takes place without physical contact with the document or article to be reproduced, as described below.
To achieve these objects, it is necessary to overcome difficulties associated simultaneously with the very high resolution required and the large dimensions of the documents in question which, as mentioned above, can be as great as a format of 3 meters (m)xc3x974 m, or even greater.
In particular, high resolution requires significant lighting of the document or article to be scanned if desired quality is to be obtained in reasonable time.
If the entire surface to be reproduced is illuminated uniformly with the appropriate light flux, (about 200,000 lux) then various drawbacks will be encountered such as:
poor contrast because the optical system and the reproduction chamber are dazzled by the light flux;
high levels of power consumption, about 50 kilowatts (kW) with the corresponding need to air condition the working premises given the considerable amount of heat generated; and
difficulty in obtaining lighting that is properly uniform over the work surface, thus making it necessary to increase the number of light sources, thereby giving rise to equipment that is relatively bulky.
Essentially, the invention proposes inputting an image by sweeping a narrow line constituted by a large number of light-sensitive points over the image plane inside a camera, and instead of illuminating the entire document, illuminating only a narrow strip in the object plane on either side of the line currently being scanned.
More precisely, the invention provides document-scanning apparatus in which a document is placed on a plane support defining an object plane, and including a camera situated at a distance from the document, the camera including an objective lens system forming an image of the document on an image plane, and the apparatus being characterized in that:
the camera includes a photosensitive sensor for scanning a line of the image plane in a first direction;
the camera includes first motor means adapted to move the sensor in controlled manner in a second direction that is distinct from the first, so as to sweep over the surface of the image plane in said second direction;
at least one light generator is provided that is suitable for forming a narrow strip of light in the object plane and extending parallel to said first direction;
the light generator includes second motor means suitable for displacing the light strip in controlled manner so as to sweep over the surface of the object plane parallel to said second direction; and
servo-control means are provided for providing servo-control between the first and second motor means to keep the scan line in the image plane in correspondence with the light strip in the object plane.
This technique presents various advantages, such as:
improved contrast (and thus improved final resolution) by not dazzling the optical system and the apparatus as a whole due to light flux corresponding to zones remote from the line currently being scanned;
energy saving, typically by a factor of about ten to about fifty compared with lighting the entire document;
no need to air-condition the working premises because less power is consumed;
greater ease in obtaining good uniformity of lighting over the work surface; and
reduced bulk compared with traditional lighting systems.
The preferred means for obtaining a narrow strip of lighting consists in providing a line generator that comprises a linear light source and a reflector in the form of a portion of an elliptical cylinder, said cylinder being defined by a director line parallel to the linear light source and by an elliptical generatrix having one of its focuses situated at the linear light source and its other focus situated in the object plane.
Amongst other things, the servo-control means can be:
cam and linkage mechanical means associating the position of the light generator with the position of the sensor; or
electronic means comprising means for determining the instantaneous position of the sensor and means for driving said second motor means in such a manner as to associate the position of the light generator with that of the sensor; or
optoelectronic means comprising means that are movable synchronously with the sensor to generate a laser beam and to project said beam on the object plane in the vicinity of the document, with distance-measuring means monitoring the respective positions of the spot formed by the beam on the object plane and the light strip and driving said second motor means in such a manner as to cause the position of the light generator to be associated with the position of the sensor.